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13.1.2 Effect of Aggregate

As noted in the previous section, the easiest way
to control the workability is by the amount of water that is added. However,
the higher the w/c, the worse the final properties of the concrete. Therefore
there is a lot of incentive to improve the workability while keeping the w/c
as low as possible. This brings us to the aggregate. The shape and surface
texture of the individual particles and the size distribution, or grading, of
the aggregate particles both affect the workability. Round, smooth particles
give the best workability, because particles with sharp corners tend to interfere
with each other as the concrete flows. The grading of the aggregate determines
how efficiently the particles pack together. If all of the particles were the
same size, then the aggregate could not occupy a very large fraction of the
concrete, because even if the particles were all touching, there would be nothing
to fill the spaces between them. However, when there is a range of particle
sizes, the spaces between the larger particles can be occupied by slightly smaller
particles, and so on.
Remember that in order to keep the cost of the
concrete down, the mix design should call for as much aggregate (i.e., as little
cement) as possible. However, it is possible to be too efficient with the aggregate
grading. If the packing of the aggregate particles is too dense, then the cement
paste cannot coat the particles uniformly and the workability will be poor.
Therefore, a tradeoff between efficient grading and workability needs to be
made. These relationships have been investigated in detail, and the best aggregate
size distributions have been worked out. This is outside the scope of this
monograph, however. For a fuller discussion of this topic see Chapter 7 of
Concrete by Mindess and Young [].
A final comment regarding the maximum aggregate
size should me made. For a given aggregate fraction in the concrete, the workability
improves as the size of the maximum aggregate particles increases. Put another
way, for a given workability the use of larger particles allows for a mix design
with less cement. However, there are other constraints on the maximum aggregate
size. First of all, when making high-strength concrete the strength is seen
to decrease with increasing maximum size. This is not a really a problem with
normal concrete, however. The most important issue is with the need for the
concrete to uniformly fill the mold as it is placed without segregating. For
this to happen, the largest aggregate particles must be able to pass smoothly
between the reinforcing bars and through the narrowest areas of the mold. Two
general rules are that the maximum size should be no more than 3/4 the size
of the space between the reinforcing bars, and 1/5 of the smallest form dimension.
Finally, it should be noted that as the maximum aggregate size increases, the
size and robustness of the mixers, pumps, and other equipment needed to make
and place the concrete also increase. For general construction such as buildings
and bridges, the maximum size is typically 1.5 inches (40 mm). For very large
projects such as dams, the size can go up to six inches.